Rome: Total War. Playing as either House of Julii and undermining other houses or going full frontal civil war on the senate. Then expanding to Carthage and Sparta. While repelling Gauls.
Medieval 2: Total War. Playing as Ottoman Empire and removing byzantine. Then repelling mongols and timurids while giving them /just/ enough way to go deeper into west so i undermine europe. Going on crusades with Teutonic Order also counts in my book.
I'm actually quite a fan some of the PS2 final fantasy games. Despite the fact FFX is a complete hormone fest i quite liked the RPG elements. One I always seem to come back to is FFXII. The gambit system was a bit on the messy side though i can't explain why but I love it.
Star Wars Bounty Hunter for PS2 had the worst controls of a game I've ever come across, but the story was downright badass.
Another one is the original Dawn of War, particularly the Dark Crusade expansion. Again, i don't know why, but I can never get enough.
Also I'll +1 the ROME TW. Shame about the sequel. Loved the fact you could rewrite the source files to open up the non-unlockable factions.
NFS Porsche and Star Wars: TIE Fighter. I know where there are HUNDREDS of 'new' missions and battles for TIE Fighter
I'll play both of those games at least once a year, pretty much without fail.
As for online games, right now it's gotta be Guild Wars 2, speaking of which, there is new 'living world' content happening soon, so when that goes live, I know what I'll be doing if I'm not in the middle of work shifts.
Rome Total War. This game has time and time again forced me to reinstall and attempt to dominate the world again yet end game always becomes huge drag due to me insisting on micro-managing each town.